(a) Field of the Invention
The present invention relates to a secondary battery, and more particularly, to a safety plate of the secondary battery, which may prevent the explosion of the battery due to an internal pressure of lithium ion secondary battery.
(b) Description of the Related Art
With the recent, widespread trend in the variety of portable electronic equipment such as high efficient notebook computers and wireless telephones, the demands on the secondary battery, which is able to be re-charged and has a large energy density, have been increased to be utilized as a power source of the equipment.
Among such a secondary battery, a lithium ion secondary battery includes a carbonaceous anode, a cathode of lithium metal oxide, a separator of polyolefin, and an electrolyte, of which charge and discharge are carried out by the electromotive force generated when lithium ions move between the cathode and the anode.
However, the lithium ion secondary battery is high in its operation electric potential, so that a high energy may flow instantly and a cathode material is largely increased in chemical activation by overcharge or short circuit, thereby the cathode material reacts with the electrolyte rapidly, generating a lot of gas.
As a result, pressure or temperature in the lithium ion secondary battery increases rapidly, bringing about an explosion of the battery, so that the environmental equipment may be damaged or users may be injured.
Therefore, various safety elements have been developed for preventing the explosion of the lithium ion secondary battery.
For example, U.S. Pat. No. 5,738,952 discloses a structure that a safety plate is connected to a cathode lead tap by welding to be turned over at a predetermined threshold pressure and the welded portion of the safety plate is detached to interrupt a current, thereby preventing the increase of the pressure and the temperature, wherein if the pressure exceeds the predetermined pressure, a wall of the battery case or a path through which the current flows is interrupted to prevent the increase of the pressure.
However, it still has disadvantages in that it is difficult to manufacture and assemble a safety plate responsive to pressure, even though the flow of a current may be prevented by interrupting the current flow from outside the battery in case of any abnormal reaction of the battery, since such an additional safety plate is mounted outside the battery case. Japanese Patent Laying-open Publication Nos. 02-284350 and 09-320549 disclose a safety element for a secondary battery that is formed of a groove in the shape of straight line or “X” formed of such straight lines on the surface of a battery case.
Such a rectilinear groove has, however, different distances to each point of the groove from the center part of a side surface which has a maximum area to be swollen the most at a predetermined threshold pressure at which the whole battery case is swollen to break, and, accordingly, pressures applied to each part of the groove become different, so that all parts of the groove are not broken simultaneously but the weakest part is broken first.
Therefore, it is impossible to form the groove to be broken simultaneously in all parts but forming the whole groove of the battery case with different thickness, since it is most preferable that the whole groove becomes broken simultaneously.
Further, it is another problem that it is difficult to design the safety element since a pressure deviation is different according to the position of the groove and the size of the battery case.
In case of the groove in the shape of “X”, it has disadvantages that it is difficult to engrave the groove on the battery case precisely and a new design is required in order to break the groove at a predetermined pressure considering the size of the battery case.
Further, the rectilinear groove and the X-shaped groove may hurt users due to the sharp broken parts of the groove, even though it is possible to prevent the explosion of the battery after being broken.